Abstract
One of the types of movements, e.g. protoplasmic movement, observed in cells are caused by the polymerization of the actin molecules. Recently, attempts to reveal the mechanism of force generation by using a Brownian ratchet model were reported. The load, e.g. the cell membrane, is pushed by the actin polymerization occurred at the head (barbed end) of the filament close to the load. Relative position between the head of the actin filament and the load are fluctuated as the Brownian motion. We focused on the association of the actin monomer at the head of the actin filament. To simplify the problem, association of the two actin monomers are modeled as the hard fused spheres and studied theoretically by using the extended scaled particle theory (XSPT) which we have developed. The mean force between two molecules is attractive as the result of the excluded volume effect.Our results by using XSPT show the probability that the force of the actin filament against the load is determined not only by the fluctuation of position of the head of the actin filament but also by the fluctuation of position of the surrounding free actin monomers moving around the head of the filament. This suggests that thermal motion of the surrounding free actin monomers is also the origins of the force of the actin polymerization motor through the excluded volume effect.
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